Seemingly everyone watches TV with some sort of second screen in front of them — be it a laptop, mobile phone or tablet device. But the tools for engaging with those viewers have largely been lacking, due to the absence of a feedback loop between the TV and second screen. San Francisco–based startup Flingo hopes to change that, with a bit of technology embedded into TVs that will let web and mobile applications know what’s being watched on the big screen.

Last year Flingo founder Ashwin Navin demonstrated for us how the technology can bring online video to the TV through white-label TV apps and an open API that any publisher can download and use itself. With that technology consumers can “fling” supported content to a queue of web videos on their TVs.

But the Flingo technology has a lot more to offer than just sending online video to the TV: It will also enable broadcasters and advertisers to build mobile and web applications that are aware of what television content you are watching. By doing so, broadcasters can provide additional relevant content to on-air shows in those applications, increasing engagement with users on the second screen.

Meanwhile, advertisers will be able to create interactive advertising using the technology. Consumers won’t have to go to a related website to find out more about a product being advertised on-screen: Clickable display ads can be surfaced in whatever browser or app they’re using while watching TV. This enables advertisers to extend offers to the second screen, and it adds a direct response mechanism to TV advertising, which has typically been just about branding and reach.

In addition to web and mobile apps, Flingo technology can be used to bring contextually relevant content directly to the TV through what it calls Hovercraft apps that take up a small portion of the screen. That will enable broadcasters to embed floating Twitter clients through the TV interface, for instance, or create interactive flash polls that can be answered on a mobile device or web browser.

Flingo isn’t the only company looking to offer this type of interactivity among TV and web and mobile apps. Yahoo offers something similar, which it calls Broadcast Interactivity, through its Yahoo Connected TV platform. Audio fingerprinting app maker Shazam is betting big that consumers will use its mobile application to identify what they’re watching and that in the process they will unlock additional content from publishers or deals from advertisers. IntoNow, which had similar technology, pitched it as a way for users to “check in” to shows with the intent of later unlocking similar content and interactive adds. IntoNow was bought by Yahoo just a few months after launch.

The beauty of what Flingo is doing is that it’s already embedded in TV chipsets, so there’s no audio fingerprinting or other workaround necessary to determine what viewers are watching. Flingo has licensed its technology to consumer electronics manufacturers like LG, Samsung, Vizio, Insignia, Sanyo and Western Digital. As a result, it’s already available on more than 5.7 million TVs, Blu-ray players and IP set-top boxes, with more being sold every day.

Most consumers will never know what Flingo is, as it will operate largely behind the scenes. But its technology could enable a whole new level of interactivity and engagement that was previously unavailable to broadcasters and advertisers.